1. Field of the Invention
The present invention relates to a developing unit that develops a latent image on a latent-image carrying member with toner, a processing unit, and an image forming apparatus.
2. Description of the Related Art
A conventional developing unit has been disclosed in Japanese Patent Application Laid-open No. 2007-133389. The developing unit includes a cylindrical toner carrying member that is rotatably held on a rotation axis.
Elongated electrodes, which extend in a linear direction of the rotation axis, are arranged on the toner carrying member at a predetermined pitch. An alternating electric field is formed between the mutually adjacent electrodes on a surface of the toner carrying member. Directions of the alternating electric field are changed such that toner hops from the top of one of the electrodes and lands onto an adjacent electrode. As a result of repeatedly hopping between adjacent electrodes, the toner is transported from one place to another place on the toner carrying member, along with rotations of the toner carrying member, and finally reaches a developing area opposite a latent-image carrying member. In the developing area, the toner, which hops from the surface of the toner carrying member and flies up to the vicinity of the latent-image carrying member, is pulled by the electric field due to a latent image and adheres to the latent image. Due to this, the latent image develops into a toner image.
In another known developing unit, the toner is transported to the developing area using a movement due to hopping instead of being transported to the developing area due to a surface movement of a toner carrying member itself. For example, a developing unit disclosed in Japanese Patent Application Laid-open No. 2004-198675 uses a toner carrying member in which three types of electrodes called A-phase electrodes, B-phase electrodes, and C-phase electrodes are repeatedly arranged in a sequence. The toner sequentially hops from the A-phase electrodes onto the B-phase electrodes, from the B-phase electrodes onto the C-phase electrodes, and from the C-phase electrodes onto the A-phase electrodes on the surface of the toner carrying member. Thus, the toner is transported towards the developing area without the toner carrying member actually rotating.
The method of hopping toner enables to realize low potential developing that cannot be realized in an existing monocomponent developing method or a bicomponent developing method. For example, the toner can be caused to selectively adhere to an electrostatic latent image where an electric potential difference between the electrostatic latent image and the surrounding non-image portion is only several tens of volts (V).
Various methods are used for supplying toner on the surface of a toner carrying member. For example, in the developing unit disclosed in Japanese Patent Application Laid-open No. 2007-133389, the toner inside a toner housing unit is transported towards a roller member due to rotatable driving of an agitator. The roller member rotates while touching a blade member. The roller member carries the toner, on the surface of the roller member itself and causes, along with self-rotation, the toner to enter a touching portion of the roller member and the blade member. The toner, which enters the touching portion, is strongly rubbed against the blade member and is friction-charged. Upon passing the touching portion along with the rotations of the roller member, the toner moves up to the vicinity of the surface of the toner carrying member and gets transferred onto the toner carrying member. Thus, the toner is supplied from the roller member to the toner carrying member.
In the developing unit disclosed in Japanese Patent Application Laid-open No. 2004-198675, a bicomponent developer, which contains toner and magnetic carrier, is caused to stick to a surface of a rotating magnet sleeve and transported up to the vicinity of the toner carrying member along with rotations of the magnet sleeve. Next, the electric potential difference between the magnet sleeve and the toner carrying member is used to transfer the toner in the bicomponent developer, which is carried on the magnet sleeve, onto the surface of the toner carrying member. Thus, the developing unit uses the bicomponent developer to supply the toner to the toner carrying member.
In developing units disclosed in Japanese Patent Publication No. H1-31611 and Japanese Patent Publication No. H4-46428, a portion of a circumferential surface of a cylindrical toner carrying member is immersed into the toner inside the toner housing unit. The toner carrying member is rotated in this state and the toner inside the toner housing unit is scooped up by the surface of the toner carrying member.
However, in the conventional technique, excessive stress is imparted on the toner. Specifically, upon carrying out various experiments using a prototype of a developing unit that uses the hopping method, the inventors discovered that hopping of the toner on the surface of the toner carrying member sufficiently friction-charges the toner. Even if the toner is not suitably charged in advance, the toner is sufficiently friction-charged due to repeated collision with the surface of the toner carrying member during the hopping until the toner is transported to the developing area. However, the developing unit disclosed in Japanese Patent Application Laid-open No. 2007-133389 charges the toner before supplying the toner to the toner carrying member. Due to this, the toner is strongly rubbed against the blade member on the rotating roller member. Such rubbing imparts unnecessary stress on the toner.
In the developing unit that is disclosed in Japanese Patent Application Laid-open No. 2004-198675 and that uses the bicomponent developer to supply the toner to the toner carrying member, generally the toner is replenished into the bicomponent developer when required and mixed by stirring. The replenished toner is slidingly rubbed with the magnetic carrier at the time of getting mixed by stirring and is friction-charged. Thus, in a structure that uses the bicomponent developer, before being supplied to the toner carrying member, the toner is inevitably slidably rubbed with the magnetic carrier in the bicomponent developer and is friction-charged. Such slidable rubbing imparts unnecessary stress on the toner.
In the developing units disclosed in Japanese Patent Publication No. H1-31611 and Japanese Patent Publication No. H4-46428, in a touching area of the toner inside the toner housing unit and the toner carrying member having the circumferential surface partially immersed into the toner, the toner is subjected to friction at the surface of the rotating toner carrying member. Such friction imparts unnecessary stress on the toner.
In the developing unit that uses the hopping method, the toner can also be supplied to the toner carrying member by using a method that is widely used in a general monocomponent developing unit. Specifically, a rotatable toner supplying member such as a sponge roller is caused to rotate while touching the toner carrying member and the toner on the toner supplying member is transferred onto the surface of the toner carrying member at a touching portion. However, in the method mentioned earlier, the toner is subjected to friction at the touching portion of the rotating toner supplying member and the toner carrying member. Such friction imparts unnecessary stress on the toner.